Remove the shelves from inside the cabinets. Set two cabinets upright on the working surface, one on top of the other. Position the drill inside the upper cabinet and make four evenly spaced holes down through the base of the upper cabinet and into the top board of the bottom cabinet. Screw 1 1/2-inch wood screws through the holes in the base of the upper cabinet, tightening them fully so the heads of the screws are flush with the base of the upper cabinet. Repeat this process for another set of two cabinets.
Position the cabinet sets parallel and next to each other with a few feet of space between them. Face both of their doors forward. Adjust the distance between the cabinets until the entire setup stretches a distance of 1 foot less than the desired bar top length. Cut out a piece of plywood that matches the width and height of the space between the cabinets.
Position the plywood piece in between the cabinet assemblies, about 3 inches in from the rear of the cabinets. Hold it in place with the help of an assistant, and predrill holes through the interior of the cabinets and into the sides of the plywood piece. Space the holes every 10 inches. Attach the plywood board to the cabinet assemblies with wood screws tightened in each premade hole.
Place a bar top board with dimensions of at least 1-by-18-by-60-inch so it spans the cabinets. Move the top board to evenly space it over the cabinets and so the front of the board is flush with the front of the cabinets. Leave some overhang of the top board over the cupboards in the rear, if possible.
Sand the top board of the bar so that the edge is comfortable for customers to lean against. Replace the shelves in the cabinets and add any additional decorations to the outside of the dry bar.